Article pubs.acs.org/EF
Production of High-Grade Carbonaceous Materials and Fuel Having Similar Chemical and Physical Properties from Various Types of Biomass by Degradative Solvent Extraction Janewit Wannapeera,† Xian Li,‡ Nakorn Worasuwannarak,† Ryuichi Ashida,‡ and Kouichi Miura*,‡ †
The Joint Graduate School of Energy and Environment, Center of Energy Technology and Environment, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand ‡ Department of Chemical Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: Eight types of biomass waste samples, microcrystalline cellulose, hemicellulose, and lignin were subjected to a degradative extraction method that treats carbonaceous resources in a non-hydrogen donor at around 350 °C. The treated products were separated at 350 °C to recover the extract and residue (the latter was termed Residue). The extract was further separated into two fractions: a fraction that precipitates as a solid (termed Deposit(s)) and the soluble fraction (termed Soluble(s)) at room temperature. The carbon-based yields of the three fractions obtained from the biomass wastes were 36.8− 71.6% Solubles, 4.4−10.6% Deposits, and 15.1−27.5% Residues. The remaining carbon was converted to CO2, CO, and a small amount of hydrocarbons that could not be separated from the solvent. Most of the ash was concentrated in the Residues, whereas the Solubles were almost completely free from ash. Surprisingly, the chemical and physical properties of the Solubles produced from the eight biomass waste samples were highly similar. The Solubles from the various biomasses had elemental compositions in the range of C = 81.0−83.3 wt %, H = 6.1−7.3 wt %, and O = 7.3−11.1 wt %. The Solubles comprised uniform low-molecularweight compounds with a peak molecular weight at around 300. The Solubles melted completely below 90 °C, and 60−70% of the Solubles were devolatilized below 400 °C. The Solubles, which constituted the largest yield fraction and have unique properties, have potential utility for various purposes.
1. INTRODUCTION Biomass waste is a renewable resource, and its effective utilization is of prime importance, particularly in Southeast Asian countries, including Thailand where massive amounts of biomass wastes are generated. Development of effective biomass conversion technologies is essential for efficient utilization of biomass. Many studies have been performed on the development of effective biomass conversion technologies, and many compilations of these technologies have already been published.1−4 Generally, the existing technologies can be categorized into thermochemical conversions such as pyrolysis, carbonization, gasification, hydrothermal liquefaction, and organosolv method, and biochemical conversions such as enzymatic hydrolysis and fermentation. Steam explosion, ammonia fiber explosion, acid treatment, etc., have also been proposed as pretreatment methods for biochemical conversions. The conversion technologies may also be categorized in terms of the products generated, such as synthetic gas, solid fuel, biodiesel, ethanol, fine chemicals, etc. Although review and evaluation of these technologies are beyond the scope of this report, most of these technologies generally produce various products in rather low yields and with rather low conversion efficiency. Furthermore, the product distribution is highly dependent on the type of biomass utilized. These drawbacks may arise from the complex structure of biomass, the high water content, and the high structural oxygen content. The authors propose that dewatering, in combination with deoxygenation of biomass under rather mild conditions prior to subjecting it to the relevant conversion process, is one of the best ways to utilize biomass effectively. A similar methodology © 2012 American Chemical Society
was effectively applied to low-rank coals. In a recent report, the authors presented a degradative extraction method that converts low-rank coals into high-quality extract.5−10 The method treats low-rank coal in a non-hydrogen donor at around 350 °C, under pressure, using a batch autoclave to dewater without phase change, to remove oxygen functional groups, and to produce low-molecular-weight compounds. The core concept underlying this method involves exposing the entire sample to thermal reactions in a nonpolar solvent at around 350 °C. The solvent is not expected to participate in chemical reactions with the samples but to act simply as a dispersant for the sample. 1-Methylnaphthalene (1-MN) was used as a solvent to meet this requirement. It is well-known and accepted in the field of direct coal liquefaction research that 1MN is not involved in chemical reactions, even in the presence of a catalyst and hydrogen, at temperatures below 350 °C. The anticipated thermal reactions under these conditions include deoxygenation reactions consisting of dehydration and decarboxylation without primary decomposition reactions accompanying the disruption of C−C bonds. 350 °C was selected as the maximum temperature to satisfy the conditions. The products formed using the proposed method involving solvent treatment at around 350 °C are then filtered at the same temperature to recover the extract and residue (the latter is called Residue). The extract is further separated into two fractions at room temperature: the fraction that precipitates as a Received: February 23, 2012 Revised: June 19, 2012 Published: June 24, 2012 4521
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Table 1. Product Yields, Ultimate and Proximate Analyses, and Element Distribution in Products ultimate analysis [wt %, daf] sample
yield [wt %, daf]
CEL Residue Deposit Soluble Gas Liquid*
4.1 4.9 28.3 8.2 54.6
HEM Residue Deposit Soluble Gas Liquid
13.4 3.3 24.5 18.1 40.8
LN Residue Deposit Soluble Gas Liquid
49.0 1.5 20.7 10.6 18.2
LC Residue Deposit Soluble Gas Liquid
12.3 3.8 36.9 17.0 29.4
EUCA Residue Deposit Soluble Gas Liquid
8.8 5.2 45.0 11.9 29.1
EFB Residue Deposit Soluble Gas Liquid
20.4 3.0 24.3 20.6 31.7
RS Residue Deposit Soluble Gas Liquid
14.6 2.3 31.5 17.0 34.0
RH Residue Deposit Soluble Gas Liquid
11.9 6.4 31.4 17.2 33.0
NP Residue Deposit
17.4 3.8
proximate analysis [wt %, db]
element balance [mol/kg-biomass, daf]
C
H
N
O*
VM
FC
ash
C
H
O
41.2 79.5 64.6 84.9
6.1 4.3 5.4 6.1
0.3 0.7 0.4 0.5
52.4 15.5 29.6 8.5
92.4 36.6 61.6 73.3
7.6 63.4 38.4 26.7
0.0 0.0 0.0 0.0
34.3 2.7 2.6 20.0 4.6 4.4
61.0 1.8 2.6 17.3 1.3 38.0
32.8 0.4 0.9 1.5 6.5 23.5
45.7 70.0 84.2 84.0
6.8 5.5 5.9 6.7
0.3 0.3 0.4 0.4
47.2 24.2 9.5 8.9
78.3 64.3 35.9 65.4
20.3 35.7 64.1 34.6
1.4 0.0 0.0 0.0
38.1 7.8 2.3 17.1 4.7 6.1
68.3 7.3 1.9 16.3 0.8 42.0
29.5 2.0 0.2 1.4 7.7 18.2
60.3 76.0 77.9 83.3
4.9 4.1 4.6 6.4
0.3 0.4 0.6 0.4
34.5 19.5 16.9 9.9
66.1 36.6 40.8 81.0
20.6 42.0 59.2 19.0
13.3 21.3 0.0 0.0
50.3 31.0 1.0 14.4 2.9 1.1
49.2 19.9 0.7 13.2 2.3 13.1
21.5 6.0 0.2 1.3 4.3 9.8
49.3 86.4 76.5 81.5
6.6 5.3 5.5 6.4
1.4 1.9 2.6 2.2
42.7 6.4 15.4 9.9
82.1 35.7 32.5 75.5
16.6 54.9 66.1 24.5
1.3 9.4 1.4 0.0
41.1 8.8 2.4 25.1 4.6 0.2
66.0 6.5 2.1 23.6 0.8 33.0
26.7 0.5 0.4 2.3 7.1 16.5
50.8 87.2 80.1 81.0
6.8 5.3 5.5 6.1
0.5 1.4 1.5 1.8
41.9 6.1 12.9 11.1
83.5 31.2 39.8 76.9
16.0 64.0 59.8 23.1
0.5 4.8 0.4 0.0
42.3 6.4 3.5 30.3 3.3 −1.2
68.0 4.7 2.9 27.4 0.5 32.5
26.2 0.3 0.4 3.1 4.9 17.4
49.8 65.8 89.5 83.3
6.6 5.6 6.2 7.2
1.6 1.8 3.2 2.2
42.0 26.8 1.1 7.3
67.9 27.6 32.9 70.5
16.7 32.0 65.9 29.5
15.4 40.4 1.2 0.0
22.9
10.5
1.9
64.7
41.5 11.2 2.2 16.9 5.1 6.1
66.0 11.4 1.8 17.5 1.0 34.3
26.3 3.4 0.0 1.1 8.9 12.8
46.2 61.7 87.5 82.3
6.7 5.0 6.2 6.8
1.6 1.7 2.9 2.0
45.5 31.6 3.4 8.9
73.0 24.0 32.1 74.8
11.9 22.8 65.8 24.4
15.1 53.2 2.1 0.8
11.2
10.6
−0.2
78.3
38.5 7.5 1.7 21.6 4.5 3.2
67.0 7.3 1.4 21.4 0.8 36.1
28.4 2.9 0.1 1.8 7.1 16.7
47.6 74.8 87.0 81.8
6.8 5.3 5.9 6.7
1.4 2.1 2.7 1.8
44.2 17.8 4.4 9.7
67.0 10.3 37.7 73.5
11.5 19.3 62.1 26.5
21.5 70.4 0.2 0.0
41.5 7.9 4.4 21.0 4.6 3.6
70.0 6.8 3.5 20.1 0.9 38.7
26.4 1.0 0.5 2.4 7.2 15.4
50.8 87.2 80.1
6.8 5.3 5.5
0.5 1.4 1.5
41.9 6.1 12.9
76.0 32.7 34.7
16.7 36.7 64.9
7.3 30.6 0.4
39.7 10.8 2.7
68.0 9.2 2.2
27.6 1.9 0.1
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Table 1. continued ultimate analysis [wt %, daf]
proximate analysis [wt %, db]
element balance [mol/kg-biomass, daf]
sample
yield [wt %, daf]
C
H
N
O*
VM
FC
ash
C
H
O
Soluble Gas Liquid
21.3 20.0 37.5
81.0
6.1
1.8
11.1
69.0
30.0
1.0
14.6 5.2 6.4
14.3 0.9 41.4
1.3 8.5 15.8
JT Residue Deposit Soluble Gas Liquid
19.0 3.0 23.6 18.0 36.5
50.6 73.1 89.1 82.5
6.9 6.3 6.5 7.3
1.4 1.5 3.1 2.1
41.1 19.1 1.3 8.1
74.4 35.8 34.7 76.8
18.7 38.1 63.7 22.2
6.9 26.1 1.6 1.0
42.2 11.6 2.2 16.2 4.6 7.5
69.0 12.0 1.9 17.2 1.1 36.8
25.7 2.3 0.0 1.2 7.6 14.6
CR Residue Deposit Soluble Gas Liquid
17.3 3.3 25.4 22.8 31.2
49.4 77.2 87.8 82.0
6.9 5.7 6.2 6.9
1.4 1.7 3.1 2.0
42.3 15.4 2.9 9.1
75.9 31.6 34.3 75.9
17.8 43.0 64.4 24.1
6.3 25.4 1.3 0.0
41.2 11.1 2.4 17.4 5.9 4.4
69.0 9.8 2.1 17.5 1.0 38.6
26.4 1.7 0.1 1.4 9.7 13.6
*
Calculated by difference.
organosolv methods. The proposed method also differs from conventional pyrolysis methods, such as fast pyrolysis, where the generated low-molecular weight compounds are devolatilized before being exposed to the highest pyrolysis temperature. Herein, the proposed degradative extraction method is applied to examination of the possibility of converting biomass wastes to raw-material-independent intermediate compounds in high yields. Toward this end, eight types of biomass wastes, a microcrystalline cellulose, a hemicellulose, and a lignin were treated using 1-methylnaphthalene as the solvent. The extracted products were characterized in detail to determine their potential utility as raw materials for production of carbon materials and high quality fuel.
solid (Deposit) and the soluble fraction (Soluble). The Soluble fraction is also finally recovered as a solid by removing the solvent. Treatment of eight brown coals using the proposed method furnished Soluble yields as high as 18.3−27.2% and Deposit yields of 3.5−16. 9% on a dry and ash free basis. Both the Solubles and Deposits were free from water and ash, and the carbon content of the Solubles was as high as 81.8−84.0 wt %. The most noteworthy finding was that the Solubles and Deposits were highly similar in terms of elemental composition,chemical structure, molecular weight distribution, thermal decomposition behavior, and thermal plastic behavior. Thus, the degradative solvent-extraction method was found to be effective for converting a wide range of low-rank coals into compounds having very similar chemical and physical properties, in rather high yields, under mild conditions. There are two well-known and interesting technologies for treatment of biomass in a solvent system at elevated temperatures: the first is the hydrothermal carbonization (HTC) method, which treats biomass under subcritical aqueous conditions,11−15 and the second is the so-called organosolv method in which biomass is treated in an organic solvent such as ethanol, methanol, acetone, and ethylene glycol, and water, with or without the use of a catalyst.16−20 It is reported that a novel carbon sphere can be prepared from saccharides such as glucose, sucrose, cellulose, and starch using HTC. The initial saccharide structure and composition, in addition to operational conditions, such as treatment temperature and holding time, may exert a significant influence on the yield and size of the carbon spheres. It is expected that physical and chemical interaction between the biomass and water should occur during the HTC treatment. The organosolv treatment is believed to hydrolyze lignin bonds and lignin−carbohydrate bonds, leaving a solid residue composed mainly of cellulose and some hemicelluloses.21,22 This method separates raw biomass into cellulose, hemicellulose, and decomposed lignin, which can be used as sources for biobased ethanol.23−25 Overall, these two interesting technologies actively utilize solvents as reactants. The proposed concept of degradative extraction, briefly introduced here, differs significantly from the HTC and the
2. EXPERIMENTAL SECTION 2.1. Materials and Solvent Used. Table 1 shows the ultimate analyses of the samples used in this study. Eight types of biomasses collected from several provinces in Thailand were used. Two of these samples comprised woody biomasses, that is, an energy crop, Leucaena (abbreviated to LC), from Saraburi province, and eucalyptus (EUCA) from a paper industry in Kanchanaburi province. The remaining six samples were either byproduct from agro-industries or agricultural residues, that is, oil palm empty fruit bunches (EFB) from Chumphon province, jatropha trunk (JT) and cassava rhizome (CR) from Nakhon Ratchasima province, rice husk (RH) from Chachoengsao province, napier grass (NP) from Nakhon Ratchasima province, and rice straw (RS) from Chachoengsao province. All biomasses were cut to less than 5 mm in length using a cutting mill. Powdered commercial microcrystalline cellulose (CEL, Merck), hemicellulose (HEM, Merck), and lignin (LN, Tokyo chemical industry) were also used for comparison purposes. The woody biomasses (LC and EUCA) were almost free from ash. On the other hand, all of the nonwoody biomasses had ash contents of more than 6.3%. Table 2 shows the structural compositions of the eight biomass samples. Significant variations in the compositions were observed from sample to sample. A non-hydrogen donor, 1-methylnaphthalene (1-MN), was used as the solvent for the degradative extraction. One significant question concerning the proposed degradative extraction method is whether 1MN is involved in the decomposition reaction of the biomass. It was clarified that the contribution of 1-MN to the decomposition reaction is negligibly small below 350 °C through detailed analysis of the 4523
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sweeping with N2 several times. The Residue remaining in the autoclave was recovered as a solid. A part of the Extract precipitated as solid at room temperature; this solid is termed Deposit(s) in this paper. The remainder of the Extract that was soluble in the solvent even at room temperature is termed Soluble(s). The Deposit was recovered as a solid by filtration using a PTFE filter (0.5 μm diameter opening). The Soluble in the filtrate was recovered as a solid by removing the solvent by rotary vacuum evaporation at 150 °C. The three obtained solid fractions were dried in a vacuum oven at 150 °C for 5−6 h. The yields of all of the solid fractions and gaseous products from the experiments were quantified. The difference represents the amount of compounds dissolved in the solvent that were removed by the rotary evaporator. These compounds are collectively called Liquid in this paper. The Liquid includes produced water and hydrocarbons dissolved in the solvent. The solid products were characterized by various analytical methods. Proximate analysis and examination of the thermal decomposition behavior were performed by using a thermogravimetric analyzer (Shimadzu, TGA50). On each run, ca. 5 mg of sample in a platinum pan (6 mm in diameter and 3 mm in height) was heated up to 920 °C in a nitrogen atmosphere at a heating rate of 10 °C/min. The elemental analysis was performed on a CHN corder (Yanaco, CHN MT-6M). The molecular weight distribution was measured by laser desorption/ionization time-of-flight mass spectrometry (Shimadzu/Kratos, KOMPACT- MALDI-II). The spectrometer was equipped with a 337 nm N2 UV laser and its acceleration voltage could be chosen as either 5 kV (low mode) or 20 kV (high mode). The laser power was carefully selected within the power range where the molecular mass distribution of the sample did not change with the laser power, in order to avoid decomposition of the sample during the measurement. Functional groups remaining in the products were qualitatively identified using a Fourier transform infrared (FTIR) spectrometer (JEOL, JIR-WINSPEC 50). FTIR spectra were acquired at 4 cm−1 resolution by averaging 64 scans in the range of 4000 to 600 cm−1 using a few milligrams of neat sample in a KBr disk. The carbontype distributions of the raw biomass and Soluble were determined for several samples using a NMR spectrometer (Chemagnetics, CMX-500 (500 MHz)) at a spinning speed of 16 kHz, contact time of 3 ms, acquisition delay of 10 μs, and pulse delay of 5 s. The softening/ melting behavior was examined by using a thermomechanical analyzer (Shimadzu, TMA). TMA was used to estimate the relative displacement of the sample when 2 mg of sample in a platinum pan (6 mm in diameter and 3 mm in height) was heated at a rate of 10 °C/ min under a load of 10 g in a nitrogen atmosphere.
Table 2. Structural Composition of Biomasses structural compositions [wt %, daf] sample (abbreviation)
cellulose
hemicellulose
lignin
extractives
leucaena (LC) eucalyptus (EUCA) oil palm EFB (EFB) rice straw (RS) rice husk (RH) napier grass (NP) jatropha trunk (JT) cassava rhizome (CR)
33.1 36.9 42.5 33.5 32.1 29.1 31.9 29.1
31.8 28.0 26.1 43.8 36.2 42.2 38.3 33.9
27.1 32.7 28.0 16.5 22.3 15.4 23.1 25.9
8.0 2.4 3.4 6.2 9.4 13.3 6.7 11.1
products and the 1-MN used for the treatment of eight kinds of brown coals.9 2.2. Experimental Procedure and Analyses. Figure 1 shows a schematic diagram of the apparatus used herein for the degradative
Figure 1. Schematic diagram of degradative solvent-extraction system. solvent extraction. The setup consists of an autoclave reactor, a reservoir, and an electric furnace. The autoclave reactor (350 mL in volume, 55 mm I.D.) was made of stainless steel, and was equipped with a magnet-driven impeller and a stainless filter plate (65 mm O.D, 0.5 μm opening) at the lower end. The autoclave reactor was connected to the stainless steel reservoir (350 mL in volume) via a valve. On each run, around 14 g of sample on a dry basis (d.b.) and 300 mL of 1-methylnaphthalene (1-MN) were charged into the autoclave reactor. Before initiation of heating, the autoclave reactor and the reservoir were purged with N2 several times, and the autoclave reactor was then sealed with 0.5 MPa of N2 and heated to the desired temperature (250−350 °C) at a rate of ca. 5 °C/min and held at the temperature for 60 min. Pressures as high as 1.8 MPa at 250 °C and 4.7 MPa at 350 °C were achieved in the autoclave reactor. The sample, together with the solvent, was stirred with an impeller throughout the heat treatment. The oxygen functional groups and weak bonds in the biomass sample were decomposed during the heat treatment, producing various products including gaseous products, water, and low-molecular-weight compounds that were dissolved in the solvent at the treatment temperature. Following heat treatment, the valve at the bottom of the autoclave reactor was opened carefully to allow the lowmolecular-weight compounds (Extract) along with the solvent to be transferred through the stainless steel filter to the reservoir, leaving the high-molecular-weight compounds (Residue) in the autoclave reactor due to the pressure difference between the reactor and the reservoir. After cooling the autoclave reactor and the reservoir to room temperature, the gaseous products were collected in a gas bag by
3. RESULTS AND DISCUSSION 3.1. Element Distributions of Products Obtained by Solvent Treatment. 3.1.1. Effect of Solvent-Treatment Temperature. In most practical applications, the product yields are represented on weight basis. To examine the changes occurring during the conversion of biomass, however, the molar basis yields are more informative than the weight basis yields. Herein, all of the yields are represented as the molar distributions of elements in the products on the basis of 1 kg of dried and ash-free (daf) biomass sample; the weight basis yields and elemental composition of each product are necessary for this determination. The yields of solid products, Soluble, Deposit, and Residue, were measured directly by weight, and the yields and compositions of gaseous products were estimated from gas chromatography. The ultimate analyses of the solid products were carried out using CHN analysis. The distribution of the elements in the solid products and gaseous products could then be calculated. The yield of Liquid could not be measured directly, but the yield and the distribution of each element in the Liquid can be estimated from the mass and elemental balances. 4524
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First, the effect of the solvent-treatment temperature on the product yield was examined at 250, 300, and 350 °C using EFB and RS. Table 3 gives the weight basis yields of the products, Table 3. Product Yields, Ultimate Analyses, and Element Distribution in Products Obtained from EFB and RS at Three Temperatures ultimate analysis [wt %, daf]
sample EFB Treated Residue Deposit Soluble Gas Liquid* Treated Residue Deposit Soluble Gas Liquid Treated Residue Deposit Soluble Gas Liquid RS Treated Residue Deposit Soluble Gas Liquid Treated Residue Deposit Soluble Gas Liquid Treated Residue Deposit Soluble Gas Liquid
yield [wt %, daf] at 250 °C 46.2 7.8 11.2 12.2 22.3 at 300 °C 25.1 4.8 19.9 18.1 31.7 at 350 °C 20.4 3.0 24.3 20.6 31.7 at 250 °C 34.3 5.2 19.2 9.8 31.3 at 300 °C 18.5 4.6 28.5 9.5 38.6 at 350 °C 14.6 2.3 31.5 17.0 34.0
elemental balance [mol/kg-biomass, daf]
C
H
N
O*
C
H
O
49.8
6.6
1.6
42.0
41.5
66.0
26.3
55.8 64.5 71.5
5.9 6.7 6.9
1.3 1.7 2.0
37.0 27.1 19.6
21.5 4.2 6.6 3.1 6.1
27.3 5.3 7.7 0.2 25.5
10.7 1.3 1.4 5.3 7.6
67.2 71.0 74.9
6.3 6.1 6.8
1.7 2.3 1.8
24.8 20.6 16.5
14.0 2.8 12.4 4.6 7.7
15.8 2.9 13.5 0.4 33.4
3.9 0.6 2.1 7.8 11.9
11.2 2.2 16.9 5.1 6.1 38.5
11.4 1.8 17.5 1.0 34.3 67.0
3.4 0.0 1.1 8.9 12.8 28.4
65.8 89.5 83.3
5.6 6.2 7.2
1.8 3.2 2.2
26.8 1.1 7.3
46.2
6.7
1.6
45.5
56.2 67.1 77.2
5.9 5.7 6.6
1.8 2.4 2.5
36.0 24.8 13.7
16.0 2.9 12.4 2.5 4.7
20.2 3.0 12.7 0.1 31.0
7.7 0.8 1.6 4.2 14.1
63.2 69.2 78.7
5.5 5.9 6.6
2.2 2.4 2.2
29.0 22.5 12.5
9.8 2.7 18.7 2.5 4.9
10.2 2.7 18.8 0.2 35.1
3.4 0.7 2.2 4.0 18.2
61.7 87.5 82.3
5.0 6.2 6.8
1.7 2.9 2.0
31.6 3.4 8.9
7.5 1.7 21.6 4.5 3.2
7.3 1.4 21.4 0.8 36.1
2.9 0.1 1.8 7.1 16.7
Figure 2. Effect of solvent treatment temperature on the element distribution in the product for EFB (a) and for RS (b).
very small. The molar ratio of hydrogen to oxygen in the Liquid fraction was roughly 2 at all of the treatment temperatures. At 350 °C, 82.5% of the structural oxygen was distributed to either the gaseous product or the Liquid. These results indicate that more than 80% of the oxygen in the samples was removed as CO2, CO, and H2O at 350 °C. The carbon in the starting material was distributed to the Soluble, Deposit, and Residue fractions at respective concentrations of 40.7%, 5.3%, and 27.0%, indicating that 73.0% of the carbon was retained in the solid product. The hydrogen was distributed to the gaseous product, Liquid, Soluble, Deposit, and Residue fractions in respective concentrations of 1.5%, 52.0%, 26.5%, 2.7%, and 17.3%. Similar changes occurred in the RS sample at lower temperatures. The amount of oxygen present in the Liquid fraction, for example, was 49.6%, even at 250 °C; this value increased to 58.8% at 350 °C. At 350 °C, 83.8% of the oxygen was distributed to either the gaseous product or the Liquid. Here again, more than 80% of the oxygen was found to be removed as CO2, CO, and H2O. The carbon was distributed to the Soluble, Deposit, and Residue fractions in respective concentrations of 56.1%, 4.4%, and 19.5%, indicating that 80.0% of the carbon was retained in the solid product. The hydrogen was distributed to the gaseous product, Liquid, Soluble, Deposit, and Residue, respectively, in amounts of 1.2%, 53.9%, 31.9%, 2.1%, and 10.9%. The thermal reactions the authors expected during the solvent treatment are deoxygenation reactions involving dehydration and decarboxylation, as stated earlier. The results shown in Figure 2 clearly demonstrate that more than 80% of the oxygen contained in the original sample was removed by deoxygenation reactions at 350 °C. The loss of carbon to gaseous products was mainly due to the formation of CO2. The largest yields among the solid products were those obtained for the Soluble fraction and their carbon-based yields reached values as high as 40.7% and 56.1%, respectively, for EFB and
*
Calculated by difference.
the elemental compositions of the solid products, and the element distributions of carbon, hydrogen, and oxygen in the products. Panels a and b in Figure 2 respectively show the element distributions in the products obtained from EFB and RS. Panel a in Figure 2 clearly shows that the oxygen distribution in the gaseous product and Liquid increased with increasing treatment temperature for EFB. The amount of hydrogen in the Liquid and the amount of carbon present in the gaseous product also increased along with these changes. Given that the main gaseous products were CO2 and CO, the amount of hydrogen distributed to the gaseous fraction was 4525
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the Deposits, and 15.1−27.5% was recovered in the Residues. In total, 70.8−95.0% carbon was recovered as solid for all of the biomass wastes. These numbers are much larger than, for example, those for the char yields obtained by pyrolysis. It was found that the elemental composition of the Soluble fractions were markedly similar, as shown in Table 1: carbon content = 81.0−83.3 wt %, hydrogen content = 6.1−7.3 wt %, and oxygen content = 7.3−11.1 wt %. This means that the rational formulas of the Solubles are CH0.894−1.062O0.065−0.114. The elemental compositions of the Solubles are deemed to be comparable to sub-bituminous coal. Although it may be of little value to discuss the elemental compositions of the Deposits given the rather small yields, the carbon contents of the Deposits were, nevertheless, also as high as 80.1−89.5 wt % and were markedly similar, with the exception of the Deposit produced from LC. These interesting results show that the proposed degradative extraction method converts different types of biomasses into Solubles and Deposits that have similar elemental compositions in rather high yields. The elemental distributions within the products for CEL, HEM, and LN are shown in Figure 3 for comparison purposes. Comparison of the product yields for the three samples shows that the largest yield of Solubles was obtained for CEL, whereas the smallest yield of Solubles was obtained for LN. The carbon distribution in the Soluble fraction was as large as 58.3% for CEL but only 28.6% for LN. The product yields for HEM were intermediate between the yields for CEL and LN. These results suggest that the differences in the elemental distributions among the eight biomass waste samples arise from the difference in the structural compositions of the samples. However, the differences in the yields among the biomass waste samples were not completely explained by the structural compositions given in Table 2. This may suggest that there is some interaction among cellulose, hemicellulose, and lignin in the biomass during the solvent treatment. 3.2. Characterization of Solid Products. In order to evaluate the characteristics of the solid products, several aspects were characterized as follows. 3.2.1. Ash Content of the Products. The ash content becomes significant when the products are utilized as fuel. Table 1 shows the proximate analyses of the solid products along with those of the raw biomass samples. It is surprising to note that the Soluble fractions are almost completely free from ash, and the ash contents of the Deposits are less than 2.1 wt %. This observation indicates that almost all of the ash was retained in the Residues. The carbon-based yields of the Residues were much smaller than those of the Solubles. It may then be acceptable to just return the Residue to soil instead of utilizing the Residue further. Based on these interesting results, the focus of the subsequent discussion is placed on the Soluble fraction that may have potential to be utilized as a high-quality fuel or raw material for carbon materials. 3.2.2. Heating Value of Solid Products. The heating value is also an essential factor for use of the solid products as fuel. Another concern is the efficiency of the degradative extraction method. The method is of little use if the heating values of the recovered products are much smaller than the heating values of the raw biomasses. The higher heating values (HHVs) of the solid products were thus examined from two separate viewpoints by estimating the heating value of each product using the following Dulong equation:26
RS. The rational formulas of the Solubles were respectively CH1.03O0.065 and CH0.991O0.083 for EFB and RS. The weight basis carbon and oxygen contents of the Solubles at 350 °C were 83.3% and 7.3% for EFB and 82.3% and 8.9% for RS, as shown in Table 3. These results clearly show that the proposed solvent treatment at ca. 350 °C is very effective for simultaneous deoxygenation and recovery of a solid product having a high carbon content and low oxygen content in rather high yield from both EFB and RS. Because the highest carbon distribution in the Deposit and Soluble fractions for both samples was achieved at 350 °C, the products obtained at this solvent treatment temperature are hereafter focused on for the remaining biomass samples. 3.1.2. Effect of Biomass Type. The weight basis yields of the products, the elemental compositions of the solid products, and the elemental distributions of carbon, hydrogen, and oxygen in the products for the eight biomasses, microcrystalline cellulose (CEL), hemicellulose (HEM), and lignin (LN) treated at 350 °C are given in Table 1. The yields and element distributions in the Liquid fractions were calculated from the mass and elemental balance, as stated. Figure 3 shows the element
Figure 3. Element distribution in the products obtained by solvent treatment of all biomass samples at 350 °C.
distributions for the products for all of the biomass samples at the solvent treatment temperature of 350 °C, including the results for EFB and RS. The element distributions within the products for the remaining six biomass wastes appear similar to those of EFB and RS at first glance. In total, 82.5−88.4% of the total oxygen was present in either the gaseous product or the Liquid fraction for all of the biomass wastes. This means that more than 80% of the structural oxygen was removed as CO2, CO, and H2O at 350 °C for all of the biomass wastes. The largest amounts of carbon (36.8−71.6%) were present in the Soluble fractions for all of the biomass wastes. Up to 71.6% of carbon was surprisingly recovered in the Soluble fraction for the woody biomass, EUCA. 4.4−10.6% carbon was recovered in 4526
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NP, JT, and CR), on the other hand, the sum of the HHVs of the solid products ranged from 78.9% to 90.8% of the HHV of the corresponding raw biomass. Some portion of the heating values was thought to be present in the Liquid fraction, as shown in Figure 3. The heating value transferred to the Liquid fractions is expected to be recovered when the Liquid is utilized by some means. These results indicate that the heating value of the raw biomass is effectively transferred to the products without significant loss using the proposed degradative extraction method. In the case of the woody biomasses, all of the heating value of the raw biomass can be transferred to the solid products without any loss. Thus, the proposed degradative extraction method was found to be effective not only for the production of a clean and highheating-value product, Soluble, and Deposit but also for transferring the heating value of the raw biomass to the products very effectively. This shows that the method is effective even when the solid product is simply utilized as fuel. 3.2.3. Molecular Weight Distributions. Figure 5 shows the molecular weight distributions (MWDs) of the raw biomasses, Deposit, and Soluble fractions measured using LD-TOFMS. It is immediately apparent that the MWDs of both the Deposit and Soluble fractions are similar, even though the MWDs of the raw biomasses were significantly different for the various samples. The MWDs of the Deposits were rather broad, having a significant molecular weight peak at around 500−600. On the other hand, the Soluble fractions consisted of smaller molecular weight compounds than those constituting the Deposits, and the MWDs of this fraction were characterized by a sharp peak at ca. 300, suggesting that the Soluble fractions comprise rather uniform compounds. These results clearly show that the Solubles exhibit potential to be utilized as raw materials in the production of value added products. 3.2.4. FTIR Analysis. To facilitate more in-depth characterization of the Solubles and in order to examine the mechanism underlying the degradative extraction, the FTIR spectra of the Soluble fractions were compared with those of the raw biomasses. Figure 6 shows a comparison of the FTIR spectra of the Solubles prepared at 250, 300, and 350 °C as well as the spectra of the raw biomass for EFB and RS. The spectra of the raw biomasses exhibit rather broad absorption bands assigned
HHV (MJ/kg, daf) = (338.1C + 1441.8H − 180.2O)/1000
(1)
where C represents the wt % of carbon, H represents the wt % of hydrogen, and O represents the wt % of oxygen. The HHVs of the Solubles and Deposits were respectively estimated to be as high as 34.1−37.2 MJ/kg and 31.0−39.3 MJ/kg. These values well corresponded to the HHV of bituminous coal on daf basis. If the fact that the Soluble fractions were almost completely free from ash and moisture is taken into account, the Soluble fraction can be judged to be a much better solid fuel than bituminous coal. This shows that the proposed degradative extraction method is effective for the production of high-quality solid fuel. The heating value recovered in the solid product was evaluated based on the sum of the HHVs of the solid products, Soluble, Deposit, and Residue fractions, and was compared with the HHV of the corresponding raw biomass on the basis of 1 kg of daf biomass, as shown in Figure 4. For the woody biomasses
Figure 4. HHVs of biomass wastes and solid products obtained from the wastes at 350 °C.
(LC and EUCA), which produced little Liquid, the sum of the HHVs of the solid products was slightly larger than the HHV of the raw sample. For the nonwoody biomasses (EFB, RS, RH,
Figure 5. Molecular weight distributions of raw biomasses (a), Deposits (b), and Solubles (c) obtained at 350 °C. 4527
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1600 cm−1. The O−C−O stretching band at 1270 cm−1, C−O stretching band at 1210 cm−1, C−O−C band at 1100 cm−1, and C−O stretching band at 1030 cm−1 became weak as the solvent treatment temperature increased. The Solubles prepared at 350 °C exhibit distinct peaks assigned to the CC stretching at 1600 cm−1, the aromatic C−H stretching at 3050 cm−1, aromatic ring at 1460 cm−1, and aromatic out-of plane C−H bend at 770 cm−1 in addition to very sharp peaks assigned to aliphatic C−H, C−H2, and C−H3 stretching at 2850−2960 cm−1 and a weakened CO stretching band at 1720 cm−1. These results clearly indicate that some aromatization reactions occurred during the degradative extraction of the biomasses in addition to the dehydration and decarboxylation reactions. These results are similar to the FTIR results reported by Pastrova et al. for the pyrolysis of microcrystalline cellulose, to produce char, at 310−390 °C under atmospheric pressure.27 Figure 7 shows a comparison of the FTIR spectra of the Solubles prepared at 350 °C and the spectra of the raw biomasses. The spectra of the raw biomasses were rather similar to each other, and the spectra of Solubles were also very similar for the eight samples, including EFB and RS. These results clearly demonstrate that similar changes occurred for all of the biomass wastes, as discussed for EFB and RS. These results also indicate that the chemical structures of the components of the Soluble fractions prepared from the eight different biomass wastes are markedly similar. 3.2.5. Solid State 13C NMR. Evaluation of whether aromatization reactions occurred during the degradative extraction was performed using 13C NMR measurements of the Soluble fractions and the raw biomass for the LC and RS samples, as shown in Figure 8. The 13C NMR spectra of both raw LC and raw RS had similar features and exhibited distinct peaks attributed to the carbons constituting cellulose: C-6 at 65 ppm, C-2, C-3, and C-5 at 73 ppm, C-4 at 85 ppm, and C-1 at 105 ppm. The peaks associated with the COOH and O−CH3 oxygen functional groups also appeared in the raw biomasses. Most of these peaks were absent from the spectra of the
Figure 6. FTIR spectra of raw biomass and Solubles obtained at 250, 300, and 350 °C for EFB (a) and for RS (b).
to O−H stretching bands (3100−3600 cm−1), C−H stretching bands (2800−3000 cm−1), and C−OH, C−O−C, and C−O stretching bands arising from cellulose (1000−1100 cm−1). The spectra of the Solubles prepared even at 250 °C were significantly different from the spectra of the raw biomasses. The main changes in the spectra of the raw biomasses compared to the Solubles prepared at 250 °C include weakening of the broad OH stretching bands in the latter, appearance of very sharp and distinct peaks attributed to aliphatic C−H, C−H2, and C−H3 stretching at 2850−2960 cm−1, CO stretching at 1720 cm−1, CC stretching at 1600 cm−1, and appearance of several other peaks attributed to aromatic moieties: C−H stretching at 3050 cm−1, aromatic ring at 1460−1520 cm−1, and aromatic out-of plane C−H bend at 770 cm−1. Rather sharp and distinct peaks attributed to O−C− O stretching at 1270 cm−1, C−O stretching at 1210 cm−1, C− O−C stretching at 1100 cm−1, and C−O stretching at 1030 cm−1 were also found in the spectra of the Solubles. The spectra of the Soluble fractions exhibit gradual changes with increasing solvent treatment temperature; the OH stretching bands became weaker, and the CO stretching band at 1720 cm−1 became weaker relative to the CC stretching band at
Figure 7. FTIR spectra of (a) raw biomasses and (b) Solubles obtained at 350 °C. 4528
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Figure 9. TG curves of raw biomasses (a) and Solubles obtained at 350 °C (b).
13
Figure 8. Solid state C NMR spectra of raw biomass and Solubles obtained at 350 °C for LC (a) and for RS (b).
Solubles, and peaks attributed to the aromatic carbons (100− 155 ppm) and aliphatic carbons (10−60 ppm) appeared in these spectra. The peaks associated with the oxygen functional groups were very weak. Pastrova et al. also reported similar changes in the 13C NMR spectra during the pyrolysis of microcrystalline cellulose at 310−390 °C under atmospheric pressure.27 These changes are also consistent with the changes observed in the FTIR spectra. The 13C NMR spectra of the Solubles appear very similar to those of bituminous coals at first glance. The fraction of aromatic carbon, fa, for raw LC and raw RS were respectively 0.127 and 0.083. These values respectively increased to 0.677 and 0.597 for the corresponding Solubles. These results clearly show that the main reactions occurring during the degradative extraction of the biomasses are significant aromatization reactions in addition to dehydration and decarboxylation reactions. 3.3. Thermal Properties of Solubles. The potential utility of the Solubles as raw materials for carbon materials was evaluated using thermal analyses. Figure 9 shows a comparison of the thermogravimetric (TG) curves, and a comparison of the thermomechanical analysis (TMA) profiles of the Solubles and the raw biomasses is shown in Figure 10. Both assays were carried out under nitrogen atmosphere at a heating rate of 10 °C/min. The TMA profile, showing the relative displacement of the sample, reaches −1.0 when the sample melts completely. The thermal properties of the samples can be discussed by comparing the TG and TMA results. Both the TG curves and the TMA profiles of the Solubles were exceedingly similar to each other, as clearly shown in both of the figures, indicating that the thermal properties of the Solubles were highly similar irrespective of the biomass type. The TG curves and TMA profiles of the raw biomasses were significantly different from those of the Solubles. The TG
Figure 10. TMA profiles of raw biomasses (a) and Solubles obtained at 350 °C (b).
curves of the biomasses are notably similar to each other below 300 °C where dehydration and decarboxylation reactions producing CO, CO2, and H2O are prevalent. The decreases in weight at temperatures above 300 °C, associated with the formation of volatile hydrocarbons, were rather small and 4529
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biomass is pyrolyzed under atmospheric pressure, a fairly large amount of volatile hydrocarbons, in addition to CO, CO2, and H2O, are produced even below 350 °C. For example, pyrolysis of cellulose yields levoglucosan (C6H10O5) as the main volatile hydrocarbon, and the char yield is as low as 0.04 on a daf basis. It is, however, well-known that low pyrolysis temperature with slow heating rates and higher pressures increases the char yield.29 The current authors also previously demonstrated that the pyrolysis of biomass under mechanical pressure increased the char yield.30 Heating cellulose up to 300 °C under 10 MPa of mechanical pressure, followed by heating to 900 °C without mechanical pressure, produced a char yield as large as 0.235 on a daf basis. This means that about one-half of the carbon was converted to char at temperatures as high as 900 °C. This high char yield was realized by minimizing the vaporization of lowmolecular-weight hydrocarbons such as levoglucosan during the heat treatment below 300 °C under 10 MPa of mechanical pressure. The high yield and low oxygen content of the Soluble fractions realized in this study may not be surprising when the results are compared with references. Herein, 1-MN was used to meet the requirement of an inert dispersion medium for the proposed method and to examine the validity of the proposed concept from fundamental viewpoints. However, the practical application of this method requires the use of a much cheaper and self-sufficient solvent than 1-MN. The use of active solvents that will chemically interact with biomass during the treatment is also not excluded given that such solvents may produce intriguing results. On the basis of these examinations, the authors can now definitely state that the proposed degradative extraction is a new method for obtaining rather uniform compounds having a peak molecular weight at around 300, in high yield, from various biomasses, although more detailed examination may be necessary to clarify the mechanism of the degradative extraction.
dependent on the biomasses. The TMA profiles of the various biomasses are apparently very different for the various biomasses at first glance. Taking into account the facts that the positions of final displacement are affected by the ash contents of the biomasses, and that the displacement temperature ranges coincide with the temperature ranges in which the weight decreases, the displacements of the biomasses are judged to be due to the weight decrease caused by the decomposition reactions. On the other hand, the temperature range for displacement of the Solubles is as low as 80−100 °C, and is lower than the temperatures at which the weight starts to decrease. These results clearly show that the Solubles undergo complete melting at temperatures at as low as 80−100 °C. This was confirmed by direct observation using an optical micrograph. The weight decreases that occur below 350 °C for the Solubles were ascribed to the devolatilization of the lowmolecular-weight compounds shown in the MWDs in Figure 5. These low-molecular-weight compounds contributed to the melting of the entire Soluble fraction at temperatures as low as 80−100 °C. The amounts of these low-molecular-weight volatile compounds in the Solubles exceeded 60 wt %. From the viewpoint of utilizing the Solubles as solid fuels, the relatively high volatile matter content should positively enhance the combustion efficiency.28 Moreover, the unique softening/ melting characteristics of the Solubles should facilitate the possibility of using the Solubles as raw materials for production of carbon materials. 3.4. Role of 1-Methylnaphthalene in the Proposed Method. The reactions occurring during the solvent treatment in 1-MN are thought to be similar to those occurring during pyrolysis in an inert atmosphere. The major differences between the solvent treatment and pyrolysis include the very high Soluble yield and low oxygen content in the Soluble fraction of the former. The core concept behind the solvent treatment method is to expose the whole sample to thermal reactions in 1-MN at around 350 °C, as stated in the Introduction. 1-MN is not expected to be involved in chemical reactions with the samples; it is expected to act as a dispersant for the sample. Here, we examine whether this postulate was actually valid for the treatment of biomass in 1-MN. The elemental distributions of the Liquid fractions shown in Table 1 were calculated based on elemental balance. These values were all positive, except for the carbon distribution of EUCA, which was as small as −1.2 mol/kg-biomass, suggesting that this might be due to experimental error. Furthermore, the H-to-O atomic ratios of the Liquids were rather close to 2:1, indicating that water is a main component of the Liquids. These good elemental balances provide adequate circumstantial evidence for excluding the involvement of 1-MN in the formation of the Soluble fraction during the solvent treatment. The solvent recovered in the rotary evaporator, which contained 1-MN and the Liquid fraction, was analyzed by GC-MS. No significant peaks, except for those of 1-MN and H2O, were detected. This is partly due to the low Liquid concentration, but it shows clearly that 1-MN was not significantly altered during the solvent treatment. The high yield and low oxygen content of the Soluble fractions were thus attributed to the exposure of the entire sample to thermal reactions in 1-MN at ca. 350 °C under pressure. Significant aromatization reactions occur under these conditions, in addition to the dehydration and decarboxylation reactions, while minimizing vaporization of the low-molecularweight hydrocarbons during the solvent treatment. When
4. CONCLUSIONS The degradative solvent-extraction method previously proposed by the authors was utilized to upgrade and fractionate eight types of biomass wastes, a microcrystalline cellulose, a hemicellulose, and a lignin using 1-methylnaphthalene as a solvent. The carbon-based yields of three fractions obtained from the biomass wastes were as high as 36.8−71.6% Solubles, 4.4−10.6% Deposits, and 15.1−27.5% Residues. The Soluble fractions, which constituted the largest yield fractions, were almost completely free from ash. The most noteworthy results were that the chemical and physical properties of the Solubles and Deposits produced from the eight biomass wastes were highly similar. The elemental compositions of the Solubles from the various biomass sources fell within the narrow ranges of C = 81.0−83.3 wt %, H = 6.1−7.3 wt %, and O = 7.3−11.1 wt %. The Solubles were composed of a uniform distribution of low-molecular-weight compounds having a molecular weight peak at ca. 300. Complete melting of the Solubles occurred below 90 °C, and 60−70% of the Solubles were devolatilized below 400 °C. The yield distributions and detailed analyses of the Solubles clarified that the reactions occurring during the degradative solvent extraction constitute aromatization reactions in addition to dehydration and decarboxylation reactions. Thus, it was clarified that the degradative solvent-extraction method proposed by the authors is a novel and effective method for recovery of significantly uniform and high quality low-molecular-weight compounds in high yield from different 4530
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biomass sources. Solubles, having such unique properties, are expected to be utilized for various purposes.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +81-75-383-2663. Fax: +81-75-383-2653. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The financial support from The Joint Graduate School of Energy and Environment (JGSEE), The Royal Golden Jubilee Ph.D. Program (RGJ), Thailand Research Fund (TRF), and National Research University Project (NRU) obtained throughout this study is gratefully acknowledged. The authors are grateful for the 13C NMR measurements that were performed at the Research and Development Center of Nippon Steel Co. Ltd.
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